The present invention relates to sensing devices and sensing circuits, which may include such sensing devices. The present invention especially relates to current sensing circuits.
The resistance of many sensor devices changes in response to a specific event. Such resistance changes can be converted by a sensing circuit to changes in voltage or changes in current, which can then be measured to determine whether the event has occurred and, potentially, the scale of the event.
Thus, FIG. 1 shows a prior art sensing circuit 100 including a known sensing device 2. The sensing device 2 may be provided as an integrated circuit (IC), which can be connected into the sensing circuit 100. The sensing circuit 100 includes a constant current source IIN connected between earth and the positive input of a voltage comparator 1. One terminal of the sensing device 2 is connected between the current source IIN and the respective, positive input of the voltage comparator. The other terminal of the sensing device 2 is earthed. The negative input of the voltage comparator 1 is connected to a reference voltage source VREF.
Before the occurrence of an event, the sensing device 2 has a predetermined resistance and, consequently, the voltage applied to the positive input of the voltage comparator is consistently above (or below) VREF. A corresponding signal VOUT (ON or OFF) is output by the voltage comparator 1. After the occurrence of an event, the resistance of the sensing device 2 is changed. This lowers (or raises) the voltage applied to the positive terminal of the voltage comparator 1 below (or above) VREF. The signal that is output by the voltage comparator 1 is therefore switched to indicate the occurrence of the event. In other words, the voltage comparator 1 implements a discrimination function based on comparison of a voltage changed by the sensing device 2 and a reference voltage VREF.
FIG. 2 also shows a prior art sensing circuit 200 including a known sensing device 2. However, in this case the positive input of the voltage comparator 1 is connected to the reference voltage VREF. The negative input of the voltage comparator 1 is connected to the output of an operational amplifier 3, which has a negative feedback resistor R. The positive input of the operational amplifier 3 is earthed. A bias voltage source VBIAS is connected to the negative terminal of the operational amplifier 3, with the sensing device 2 connected in series between the bias voltage source VBIAS and the negative terminal.
Before the occurrence of an event, the sensing device 2 has a predetermined resistance and a corresponding current is supplied to the negative input of the operational amplifier 3. The operational amplifier 3 provides current to voltage conversion gain and a corresponding voltage is therefore applied to the negative terminal of the comparator. This corresponding voltage is consistently above (or below) the reference voltage VREF and a corresponding signal VOUT (OFF or ON) is output by the voltage comparator 1. After the occurrence of an event, the resistance of the sensing device 2 is changed. This lowers (or raises) the current supplied to the negative terminal of the operational amplifier 3. Hence, the voltage applied to the negative terminal of the voltage comparator 1 is lowered below (or raised above) VREF. The signal VOUT output by the voltage comparator 1 is therefore switched to indicate the occurrence of the event. Again, the voltage comparator 1 implements a discrimination function. However, this discrimination is performed by converting the current of the sensor device 2 to a voltage and then comparing that voltage with a reference voltage VREF.
It is often necessary to amplify the signal further for subsequent stages of electronics.
In the circuits of both FIGS. 1 and 2, the change in the resistance of the sensing device 2 is converted and compared with a constant threshold voltage signal VREF to discriminate the occurrence of an event. However, in practice the output of the sensing device is dependent not only on a specific parameter related to the event but also on other factors such as temperature and ageing. In addition, a tolerance exists in the characteristics of the sensing devices due to the fabrication process technology and/or variations in process conditions.
As shown in FIGS. 1 and 2, a differential voltage comparator 1 is commonly used to implement the discrimination function. A typical differential circuit for implementing the differential voltage comparator 1 is shown in FIG. 3. Specifically, the differential circuit includes a current mirror pair of transistors T101, T102, each connected to a first common rail VDD. The gates of the current mirror pair of transistors are connected to each other, and the drain of one of the second current mirror transistors T101 is connected to its gate.
In addition, the differential circuit includes an input or differential pair of transistors T103, T104, each connected in series to a respective one of the current mirror gates of the input pair of transistors are connected to input voltages VIN1 and VIN2. Specifically, when used in the sensing circuit of FIG. 1, the gate of input transistor T104 would be connected to current source IIN and the gate of input transistor T103 would be connected to reference voltage VREF. The voltages applied to the gates of the input transistors T103, T104 determine the current passed by the input transistors T103, T104.
The differential circuit further includes a current source transistor T105, connected in series between a second common rail VSS and a node, which is itself connected to both the input transistors T103, T104. The gate of the current source transistor T105 is biased to saturation so that a uniform, constant current is provided to both the matched input transistors T103, T104. However, the current that the input transistors pass is determined by the input voltages VIN1 and VIN2.
The output voltage VOUT of the differential circuit is the voltage at a node between the current mirror transistor T102 and the input transistor T104.
As shown in FIG. 4, an output amplification stage may be added to the differential circuit shown in FIG. 3. The output amplification stage includes transistors T106, T107 between the common rails VDD and VSS, in series with each other and in parallel to the other transistors T101-T104. The voltage at the node between the current mirro transistor T102 and the input transistor T104 of the differential circuit is the input voltage to the gate of transistor T106 and the gate of transistor T107 is separately controlled. In this case, it becomes an operational amplifier as shown in FIG. 4.